Miniature dipole E-field probes for characterizing both phase and amplitude of microwave radiators for hyperthermia.

This paper describes a system for characterizing th electric field patterns of microwave radiators in lossy media, in particular, of those used in hyperthermia treatments for cancer therapy. We discuss the design, fabrication and testing of small, minimally-perturbing electric field probes which are capable of measuring both amplitude and phase. An appropriate test configuration for mapping field patterns radiating from hyperthermia applicators (antennae) also is described. The system was developed specifically for the evaluation of applicators centered at 434 MHz and 915 MHz.

Current sheet applicator arrays for superficial hyperthermia of chestwall lesions.

The current sheet applicator is an electromagnetic heating device whose size may be chosen virtually independent of frequency even though practical limitations may restrict it to VHF and UHF bands. In this paper we investigate absorbed power distributions in muscle tissue from current sheet applicators when used as elements of a planar array intended for superficial hyperthermic treatment of tumours. Advantages offered by current sheet applicators for tissue heating include compact size, a linear polarization of the induced electric field and relatively large heating area. It is shown that the effective field produced by a pair of these elements is continuous regardless of whether the common edges of the elements are perpendicular or parallel to the direction of impressed current. The feasibility of customizing the shape and size of the field is also illustrated. The absorbed power distribution patterns due to a coherently driven array operating around 434 MHz is relatively insensitive to phase variations of about 20 degrees but is sensitive to relative power level variations as low as 10%. Mutual coupling between array elements may be reduced to acceptable levels by incorporating suitable spacing between them. It is also demonstrated that there is good agreement between measurements of absorbed power distributions and predictions using the Gaussian beam model.

Use of Gaussian beam model in predicting SAR distributions from current sheet applicators.

The Gaussian beam model is shown to be a good predictor of SAR distributions due to current sheet applicators (CSAs). It is fast, efficient and adaptable. SAR distributions from a single applicator and from simple arrays of CSAs in homogeneous and layered lossy media are computed at 434 and 450 MHz at CPU times of less than 60 s. The good agreement between theory and experiment justifies the use of the Gaussian beam model to predict SAR distributions from CSAs.

The CDRH Helix-I: a physical evaluation.

The use of a resonant helical coil with predominantly axial electric fields for regional hyperthermia in the abdomen and pelvis is addressed. The Helix-I applicator, which consists of a three-turn, 36-cm-long, oval-wound helical coil measuring 60 and 43 cm along its major and minor axes, respectively, is described, and specific absorption rate (SAR) measurements for the device are reported. Measurements of the E-field are also described. Specific absorption patterns for the Helix, determined by transient temperature measurements using a gel phantom, and by E(2)-field scans using a liquid phantom, are in general agreement. The general agreement of electric power intensity distribution, inside and outside the phantom, with corresponding SAR distributions obtained from transient temperature data, ensures reliability of the distribution patterns. The E(2) contours provide a clear picture of hot and cold spots as well as the nature of the general distribution.